It is always a process of elimination starting from the bottom up. You must gradually remove sections of the "power train" until you isolate the cause. "Squeak...grunt...grind...rub...chirp, etc" are all different types of friction sounds. There may also be more than one cause.
If it is where you think it is, it will require some disassembly. If you're not familiar with it, best to bring in someone experienced in this.
The fact that you have been temporarily successful at quieting the sound indicates that you are on the right track, but may very treating it incorrectly. Also, don't be fooled simply because it is a new piano. If you bought it from a dealer and it is still within warranty you should be having the dealer fix it at no cost (I think).
Peter Grey Piano Doctor
Thank you! It was actually quite easy to find where the sqeaks originate from, the cause is harder but I guess it’s from normal wear. The piano is quite old, at least 50 years, probably more. It’s also easy to just remove the whole mechanics part and to reach the sqeaky area.
My main question is what you normally would lubricate this with? Metal parts, wooden parts, fabric, leather etc? Is my idea about PTFE Dry Lube spray or some kind of stearic candle flour on the right track?